STN LCD panels are composed of many pixels that can either be on or off at any given time. The panel is made up of x number of pixels per line, and have y number of lines per panel. Updating all the pixels on all the lines of a panel constitutes one frame of data. Since each pixel for an STN panel has two states (on and off), each pixel is able to achieve two grayscale values, black and white. To achieve more (perceived) grayscale values, the pixels can turn on and off at a very high rate. Because the human eye is unable to detect this high rate of switching, the resulting grayscale value is somewhere between black and white, thereby giving it an apparent or perceived grayscale value.
One consequence associated with this high rate of switching is known as flickering. Flickering is a phenomenon that results in the human eye perceiving that the panel display is pulsating when the display should be uniform. This phenomenon can be distracting and undesirable.
There are various types of flickering in LCD panels (e.g., single pixel flickering and adjacent pixel flickering). Single pixel flickering can occur if the on/off time has a low frequency. Adjacent pixel flickering can occur when pixels in the same proximity are controlled according to identical schedules. However, when single pixel flickering occurs, those pixels with detectable switching appear to pulsate. When adjacent pixel flickering occurs, areas of the panel appear to pulsate.
The idea of grayscale shading with STN LCD panels is based on the principle that if power to a pixel is oscillated fast enough, the human eye will be unable to perceive the oscillation, and the person will only see the intended shade. One method of applying this theory is to divide a time segment into, for example, 16 parts. To achieve a particular shade, the pixel may be turned on for a predetermined fraction of the time segment.
As a nonlimiting example, if the desired shade of a pixel at a particular point in time is one half of full power, the desired shade value could be assigned a value of 8. If there are 16 possible grayscale values (i.e., the time segment was broken up into 16 parts), then the corresponding denotation is 8/16. To achieve this shade, the pixel may be held on for the first 8 counts, and held off for the last 8 counts.
With respect to single pixel flicker, the problem results when a pixel's state is held for a duration such that the switching is detectable by the human eye. Referring to the previous nonlimiting example, holding a pixel in one state for 8 counts may enable a person to perceive when the pixel switches states. If this occurs, the pixel will appear to pulsate. Of course, pulsating effect can be reduced, by turning the pixel on and off every other count.
Adjacent pixel flickering is a phenomenon that results when multiple pixels on a display are oscillated according to identical schedules within the given time segment. In keeping with the previous example, suppose a pixel were turned on for 8 (out of 16) counts, and it was turned on and off with every other count, such an approach would avoid single-pixel flicker, but if all pixels were turned on and off at the same count, then adjacent pixel flickering could be observed.
Another phenomenon that should be taken into account when designing an STN LCD panel is that the human eye detects brightness in a nonlinear fashion. Thus, a small change in brightness at a dark grayscale is less noticeable than equal change in brightness at a bright grayscale. Therefore, designing an STN LCD panel with a linear shade distribution is less than effective in portraying all possible shades to the observer.
In designing an STN LCD panel, a frame rate control block (FRC) is often desired. A simple method for designing an FRC is to have a frame counter that counts from 0 to 15 and then restarts. The decision to turn one pixel on at a given frame may be based on the simple pseudo-code:
If (data[3:0] >= counter), then output = 1, else output =0where “data [3:0]” is the grayscale value and “counter” isthe current value in the frame counter.
This technique is oftentimes too simplistic and may cause single pixel flicker as well as adjacent pixel flicker.
Accordingly, there is a heretofore unaddressed need to overcome the aforementioned deficiencies and shortcomings.